Measurement receivers are used in a variety of applications, including spectrum analyzers, vector signal analyzers, and wireless test sets. Noise generated by the electrical components within the receivers limits the dynamic range of the measurements and alters the test results. Subtracting the noise from a measurement signal is therefore desirable in order to generate reliable results.
One technique for noise subtraction calculates the amount of noise present in a receiver when the receiver is in a particular state. For example, the noise may be determined when the receiver is in a maximum gain state. A sample of the instrument noise is measured when the device under test (DUT) is disconnected from the test equipment. The DUT is then connected to the test equipment and the noise subtracted from a measurement signal output by the DUT.
Typically the amount of noise measured in a particular state applies only to that state. This means the noise subtraction process must be repeated every time the receiver is set to a new state, such as, for example, a minimum gain state. The user must interrupt his or her test or measurement procedure and perform noise subtraction for each new receiver state.